1. Field of the Invention
The present invention relates to a pouch of a battery and a pouch type secondary battery having the pouch, and more particularly, to a pouch of a battery having a space for containing an electrode assembly, the space being formed in a vessel form, and a pouch type secondary battery using the same.
2. Description of the Related Art
In general, secondary batteries are rechargeable batteries, and the secondary batteries include nickel-cadmium batteries, nickel-hydrogen batteries, lithium batteries and the like. Among the secondary batteries, lithium batteries have an operating voltage of 3.6 V, which is three times higher than nickel-cadmium batteries used as power sources of the electronic devices. Also, the lithium batteries have other excellent characteristics, for example, high energy density per unit weight.
Lithium batteries may include a liquid electrolyte battery and a polymer electrolyte battery, depending on the types of electrolytes of the batteries. In general, batteries using liquid electrolyte are referred to as lithium secondary batteries, and batteries using polymer electrolyte are referred to as lithium polymer batteries. In addition, the lithium secondary battery may be manufactured in various shapes. For example, the cases of the lithium liquid secondary batteries are generally made as cylindrical shape and angular shape, the cases of the lithium polymer batteries and the like are generally made in pouch shape. In other words, Lithium liquid secondary battery has three shapes, (angular, cylindrical and pouch shapes) according to the type of its case.
As briefly described herein, the pouch type lithium battery includes a pouch as a case, and an electrode assembly is contained by the pouch.
The electrode assembly includes: a cathode plate in which a surface of an electrode collector is coated by a cathode active material; an anode plate in which a surface of an electrode assembly is coated by an anode active material; and a separator disposed between the cathode plate and the anode plate to prevent a short circuit generated by the contact of the anode plate with the cathode plate. The separator is generally formed of polymers, the separator is formed of polymers and prevents a short circuit due to the physical contact of the anode plate and the cathode plate, and the separator simultaneously enables movement of lithium ions through a transferable medium for the lithium ions between the cathode active material and the anode active material, the cathode active material and the anode active material are respectively deposited on the cathode plate and the anode plate.
The pouch may be formed to contain an electrode assembly between an upper pouch membrane and a lower pouch membrane, and to seal the peripheries of the upper pouch membrane and the lower pouch membrane. Electrode taps are exposed outwardly through sealing portions, and the electrode taps are electrically coupled to the respective electrode plates of the electrode assembly to electrically couple the electrode assembly to external electrodes.
In the pouch as configured above, the upper pouch membrane may be referred to as a top of the pouch, and the lower pouch membrane referred to as a bottom of the pouch. The bottom of the pouch may be manufactured in a vessel form to hold an electrode assembly. Here, the bottom of the pouch may be manufactured in a vessel form by a deep drawing process using a press and a punch.
The bottom of the pouch manufactured in a vessel form is provided with a bottom surface corresponding to the bottom of a vessel, and lateral surfaces corresponding to side wall surfaces of the vessel, and the upper portion of the vessel form is open to form an opening. The edge, which is a section where the lateral surface is in contact with the bottom surface, is formed approximately at a right angle.
The procedural steps for forming a pouch type secondary battery, however, generally include an injection of liquid electrolyte into a pouch, an initial charging/discharging, and a removal of gases generated during the initial charging/discharging while an electrode assembly being held in an inner space of the pouch. In particular, both of the injection of liquid electrolyte and the removal of gases is performed under vacuum condition. Because the electrode assembly is generally smaller than the inner space disposed in the pouch, the pouch is contracted by the external pressure. This contraction is similar to pouches used for packaging foods in vacuum, when the inner space of the pouches are vacuumed. In particular, a distance between a bottom surface of the bottom of the vessel type pouch and the top of the pouch becomes smaller, when the bottom surface and the top of the pouch are subject to the external pressures. Since the area of bottom surface is larger than the area of the lateral surfaces, total force applied on the bottom surface is larger than on the lateral surface when a same external pressure is respectively applied to the bottom surface and lateral surface.
A distance between the bottom surface and the top of the pouch, however, is sustained by mean of the lateral surfaces in connection to a peripheral region of the bottom surface of the pouch bottom, since the lateral surfaces act as a vessel type backbone of the pouch bottom. In particular, the peripheral region has high characteristics to sustain the original height, because a section elongated during a vessel type pouch is formed in the deep drawing process, is mechanically stronger and more rigid than other sections.
Meanwhile, there is no support means within the central region of the bottom surface of the bottom of the pouch, and therefore the thickness of the central region may be reduced to a thickness of the electrode assembly. As a result, the periphery of the pouch significantly extrudes compared to the central region of the bottom surface, since the thickness of the periphery coupled to the side walls is sustained by the lateral surfaces. The extruded portion forms an available space therein or a folded portion, which may increase the entire thickness of the pouch type secondary battery. And the uniformed thickness of the pouch may cause detachment of active materials of the electrode assembly received by the pouch.